Electroplating of aluminum can be carried out with difficulty using a plating bath of an aqueous solution type, since aluminum has a large affinity for oxygen and shows a lower potential than hydrogen. For this reason, the electroplating of aluminum has hitherto been carried out using a plating bath of a non-aqueous solution type, in particular, a plating bath of an organic solvent type.
In regard to the plating bath of this organic solvent type, available as a plating bath that secures safety in operations is a molten salt bath comprised of an aluminum halide and an N-alkylpyridinium halide. It includes, for example, a molten salt bath comprised of an aluminum halide and an N-ethylpyridinium halide or a bath obtained by mixing an organic solvent in this bath (U.S. Pat. Nos. 2,446,331; 2,446,349; and 2,446,350). and a molten salt bath comprised of an aluminum halide and an N-butylpyridinium halide, or a bath obtained by mixing an organic solvent, capable of obtaining products having better appearance than those obtained by using the above plating baths even if the plating is carried out in a high current density (Japanese Unexamined Patent Publications No. 70592/1987 and No. 70593/1987, or U.S. Patent application No. 092,517/87).
This molten salt bath, though sometimes bringing about coatings partly having a problem in appearance, comes to a liquid near room temperature and makes it possible to carry out electroplating in the temperature range of approximately from 0.degree. to 150.degree. C. when a compound comprising a halogen atom selected from Cl, Br or I is used as the aluminum halide and a compound whose N-substituted alkyl group has 1 to 5 carbon atoms is used as the N-alkylpyridinium halide in amounts of from 40 to 80 mol % for the aluminum halide and from 20 to 60 mol % for the N-alkylpyridinium halide.
U.S. Pat. No. 2,446,349 discloses that the above molten salt bath dislikes water and oxygen, and presence of these results in oxidation of the bath to lower the quality of plating, thus requiring to bring its atmosphere into a dry and oxygen-free atmosphere. For this reason, also when a metal strip is continuously plated, it is necessary to take care that the water adhered on the strip in the course of a pretreatment of the plating may not be carried in the bath.
Since, however, when oxides exist on the strip, the oxides can not be removed unless an inorganic acid is used, the pretreatment must be carried out using a treatment solution of an aqueous solution type. This pretreatment is commonly carried out through the steps of degreasing, washing with water, pickling using an inorganic acid, washing with water, and, as occasion demands, strike plating, but drying is required before plating to prevent the water from being carried in the bath. This drying, because of an activated state on the surface of the strip, must be carried out in the dry and oxygen-free atmosphere before a good deposit adhesion can be obtained.
A generally available method for giving the dry and oxygen-free atmosphere is a method in which nitrogen gas or argon gas is used. However, this method, though not requiring to use gas in a large quantity in carrying out the plating in an experimental scale since there is less evaporation of water from the strip, requires to use gas in a large quantity in carrying out the continuous plating since there occurs the evaporation of water in a large quantity. Hence, in the case of the continuous plating, a step for carrying out drying in an atmosphere has hitherto been provided between the washing after pickling and the plating in a bath. This method, however, have had the problem that because of the activated state on the surface of the strip an oxide coating is formed thereon even if the drying is carried out in a short time, resulting in a lowering of the deposit adhesion.
Also, in recent years, high-purity aluminized metal sheets have attracted notices as electronic parts such as IC lead frames and materials for magnetic discs, but plating on such products requires high purity of the bath. However, the aluminum halide to be used contains a trance amount of impurities such as Fe, Pb and H.sub.2 O even if it is of high purity, and hence has been inevitably accompanied with inclusion of impurities. It has hence been difficult to attain a 99.9% or more Al purity of coatings, and moreover, the impurities may give a greater influence if the plating is carried out at a bath temperature of 40.degree. C. or more, resulting in no denseness of the coatings.
This have caused the problems that employment thereof in the IC lead frames may result in a poor bonding property to aluminum wires and working thereof into the magnetic discs may result in no capability of precise bore working by anodizing treatment.
A first object of this invention is to provide a process for the electrodeposition of aluminum, that can activate the surface by an inexpensive means even if the drying is carried out in an atmosphere after the pretreatment with use of a treatment solution of the aqueous solution type, when aluminum is continuously plated on the metal strip.
A second object of this invention is to provide a process for the electrodeposition of aluminum, that enables aluminum plating with a 99.9% or more purity.
A third object of this invention is to further provide a process for the electrodeposition of aluminum, that enables aluminum plating with a 99.9% or more purity and with denseness, even when the plating is carried out at a bath temperature of 40.degree. C. or more.